1. Field of the Invention
This invention relates to stepper motors and particularly to rotary stepper motors of the variable reluctance (also called synchronous inductor) type and a method of operation therefor.
2. Description of the Prior Art
The rotary variable reluctance stepper motor of the type to which the present invention is related involves a stator having plural poles, inductive windings on the stator poles, and a cylindrical rotor of magnetic material whose rotation is controlled by the rate of change in the sequence of energization of the windings. A characteristic of this type motor is that the pole faces adjacent the rotor and the rotor surface are provided with teeth. To cause the rotor to rotate with a steplike manner, the pitch to the rotor teeth differs by a predetermined ratio from the pitch of the stator teeth.
For many applications of the stepper motor it is desirable or essential that the motor be rotated in increments or steps of precisely uniform amounts. This has not always been achieved with the desired degree of success since the dynamic response for any given step was not always optimum. Various structures and techniques for energizing the windings have been devised. Examples of such techniques are shown in U.S. Pat. Nos. 3,535,604, issued to Elmer W. Madsen and Hermann Rosshirt on Oct. 20, 1970, titled "Electric Stepping Motor", and 3,509,392, issued to Arthur E. Snowden on Apr. 28, 1970, titled "Variable Reluctance Electric Stepping Motor with Field Energizing Circuitry" and prior art cited therein.
Heretofore as discussed in the above-mentioned Madsen and Snowden patents, it was taught that exact step operation in a variable reluctance motor required a magnetic structure and energization arrangement in which the individual poles would always be magnetized with the same polarity. Among other things this approach to the problem of uniform incrementing of the motor required a relatively large number of poles, particularly where polyphase energization is desired. For example, for two-phase energization a four phase 8 pole motor is required. This made the motors relatively large in size thereby increasing the amount of energy needed to drive them. At the same time, the amount of torque obtainable was not high enough.